Method for effecting gas-liquid contact

ABSTRACT

Viscous liquids, such as liquid sulfur and bitumen asphalt are contacted with gases to strip absorbed gases from the viscous liquid and/or transfer gaseous components into the viscous liquid using a shrouded impeller combination immersed in the viscous liquid. The invention has particular application to the stripping of hydrogen sulfide and hydrogen polysulfides from liquid sulfur using an oxidizing gas.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.08/313,153 filed Apr. 5, 1993 now U.S. Pat. No. 5,520,818, whichapplication is a continuation-in-part of U.S. patent application Ser.No. 863,720 filed Apr. 3, 1992 (now U.S. Pat. No. 5,352,421), whichitself is a continuation-in-part of U.S. patent application Ser. No.7622,485 filed Dec. 5, 1990 (now U.S. Pat. No. 5,174,973) which itselfis a continuation-in-part of U.S. patent application Ser. No. 08/582,423filed Sep. 14, 1990 (now abandoned) which itself is acontinuation-in-part of U.S. patent application Ser. No. 446,776 filedDec. 6, 1989 (now abandoned).

FIELD OF INVENTION

The present invention relates to method and apparatus for effectinggas-liquid contacting, for example, for the removal of components fromgas streams, in particular by chemical conversion or removal of gaseouscomponents while in contact-with a liquid phase or slurry, or for theremoval of components from a liquid phase, in particular, for thechemical conversion or removal of components from a liquid phase whilein contact with the gaseous phase.

BACKGROUND TO THE INVENTION

Procedures for effecting contact between a gas phase and a liquid phasehave been devised for a variety of purposes, for example, to remove acomponent from the gas or liquid phase. In this regard, many gas streamscontain components which are undesirable and which need to be removedfrom the gas stream prior to its discharge to the atmosphere or tofurther processing. One such gaseous component is hydrogen sulfide,while another such component is sulfur dioxide.

Hydrogen sulfide occurs in varying quantities in many gas streams, forexample, in sour natural gas streams and in tail gas streams fromvarious industrial operations. Hydrogen sulfide is odiferous, highlytoxic and a catalyst poison for many reactions and hence it is desirableand often necessary to remove hydrogen sulfide from such gas streams.

There exist several commercial processes for effecting hydrogen sulfideremoval. These include processes, such as absorption in solvents, inwhich the hydrogen sulfide first is removed as such and then convertedinto elemental sulfur in a second distinct step, such as in a Clausplant. Such commercial processes also include liquid phase oxidationprocesses, such as Stretford, LO-CAT, Unisulf, SulFerox, Hiperion andothers, whereby the hydrogen sulfide removal and conversion to elementalsulfur normally are effected in reaction and regeneration steps.

In Canadian Patent No. 1,212,819 and its corresponding U.S. Pat. No.4,919,914, the disclosure of which is incorporated herein by reference,there is described a process for the removal of hydrogen sulfide fromgas streams by oxidation of the hydrogen sulfide at a submerged locationin an agitated flotation cell in intimate contact with an iron chelatesolution and flotation of sulfur particles produced in the oxidationfrom the iron chelate solution by hydrogen sulfide-depleted gas bubbles.

The combustion of sulfur-containing carbonaceous fuels, such as fueloil, fuel gas, petroleum coke and coal, as well as other processes,produces an effluent gas stream containing sulfur dioxide. The dischargeof such sulfur dioxide-containing gas streams to the atmosphere has leadto the incidence of the phenomenon of "acid rain" which is harmful to avariety of vegetation and other-life forms. Various proposals have beenmade to decrease such emissions.

A search in the facilities of the United States Patent and TrademarkOffice with respect to gas-liquid contacting procedures has revealed thefollowing United States patents as the most relevant-.to the presentinvention:

    ______________________________________                                        U.S. Pat. No. 2,274,658                                                                          U.S. Pat. No. 2,294,827                                    U.S. Pat. No. 3,273,865                                                                          U.S. Pat. No. 4,683,062                                    U.S. Pat. No. 4,789,469                                                       ______________________________________                                    

U.S. Pat. Nos. 2,274,658 and 2,294,827 (Booth) describe the use of animpeller to draw gas into a liquid medium and to disperse the gas asbubbles in the liquid medium for the purpose of removing dissolvedgaseous materials and suspended impurities from the liquid medium,particularly a waste stream from rayon spinning, by the agitation andaeration caused by distribution of the gas bubbles by the impeller.

The suspended solids are removed from the liquid phase by frothflotation while the dissolved gases are stripped out of the liquidphase. The process described in this prior art is concerned withcontacting liquid media in a vessel for the purpose of removingcomponents from the liquid phase by the physical actions of strippingand flotation.

These references Contain no discussion or suggestion for removal ofcomponents from gas streams by introduction to a liquid phase or thetreatment of components dissolved or suspended in the liquid phase bychemical interaction with components of the gas phase. In addition, thereferences do not describe any critical combination of impeller-shroudparameters for effecting such removal, as required herein.

U.S. Pat. No. 3,273,865 describes an aerator for sewage treatment. Ahigh speed impeller in the form of a stack of flat discs forms a vortexin the liquid to draw air into the aqueous phase and circulate theaqueous phase. As in the case of the two Booth references, this priorart does not describe or suggest an impeller-shroud combination foreffecting the removal of components from a liquid phase or gaseousphase, as required herein.

U.S. Pat. No. 4,683,062 describes a perforated rotatable body structurewhich enables liquid-solid contact to occur to effect biocatalyticreactions. This reference does not describe an arrangement in whichgas-liquid contact is effected.

U.S. Pat. No. 4,789,469 describes the employment of a series of rotatingplates to introduce gases to or remove gases from liquids. There is nodescription or suggestion of an impeller-shroud combination, as requiredherein.

In addition, during the course of prosecution of pre-cursor U.S. patentapplications, a number of other references has been cited, identified insuch filings. The relevance of such prior art is discussed in suchprosecution.

Many other gas-liquid contactors and flotation devices are described inthe literature, for example:

(a) "Development of Self-Inducing Dispenser for Gas/Liquid andLiquid/Liquid Systems" by Koen et al, Proceeding of the Second EuropeanConference on Mixing, 30 th Mar.-1 st Apr. 1977;

(b) Chapter entitled "Outokumpu Flotation Machines" by K. Fallenius, inChapter 29 of "Flotation", ed. M. C. Fuerstenau, AIMM, PE Inc, New York1976; and

(c) Chapter entitled "Flotation Machines and Equipment" in "FlotationAgents and Processes, Chemical Technology Review #172", M. M. Ranney,Editor, 1980.

However, none of this prior art describes impeller-shroud structure usedherein.

SUMMARY OF THE INVENTION

In the present invention, a novel procedure is provided for effectinggas-liquid contact between a gas and a liquid which employs a rotaryimpeller and shroud combination operated under specific conditions toeffect rapid mass transfer between gaseous and liquid phases andexcellent agitation of the liquid phase, by the pumping action of therotary impeller, and thereby achieve an enhanced efficiency of removalof a component from the gas or liquid phase or transfer of a componentfrom one phase to the other by chemical reaction, adsorption, absorptionor desorption.

In one aspect of the present invention, there is provided a method forthe distribution of a gaseous phase in a liquid phase using a rotaryimpeller comprising a plurality of blades at a submerged location in theliquid phase surrounded by a shroud through which are formed a pluralityof openings.

The impeller is rotated about a substantially vertical axis at thesubmerged location within the liquid phase at a blade tip velocity of atleast about 350 in/sec (at least about 9 m/s), preferably at least about500 in/sec (preferably at least about 12.5 m/s), for example, up to andgreater than about 700 in/sec (up to and greater than about 18 m/s), anddraws liquid phase to the interior of the shroud effecting vigorouscirculation of liquid phase through the impeller and shroud combination.

A gaseous phase is fed to the submerged location and the shear forcesbetween the impeller blades and the plurality of openings in the shrouddistributes the gaseous phase in the liquid phase as bubbles to theinterior of the shroud and to form a gas-liquid mixture of bubbles ofthe gaseous phase in liquid phase contained within the shroud and toeffect intimate contact of gas and liquid phases at the submergedlocation while effecting shearing of the gas-liquid mixture within theshroud and initiating and sustaining rapid mass transfer.

The gas-liquid mixture flows from within the shroud through and incontact with the openings therein to external of the shroud at a gasvelocity index (GVI) of at least about 18 per second per opening,preferably at least about 24 per second per opening which causes furthershearing of the gas liquid mixture and further intimate contact ofgaseous phase and liquid phase.

The gas velocity index (GVI) is determined by the expression: ##EQU1##where Q is the volumetric flow rate of gas (M³ /s), n is the number ofopenings in the shroud, A is the area of the opening (m²) and P is thelength of the perimeter of the opening (m).

An important operating parameter of the process is the relationship ofthe shroud diameter relative to the impeller diameter for a given GVIand impeller tip speed and this parameter may be termed the EffectiveShear Index (ESI). The ESI is determined by the expression: ##EQU2##wherein GVI is the gas velocity index (/s), V_(i) is the impeller bladetip velocity (m/s), and D_(s) and D_(i) are inside diameter of theshroud and outside diameter of the impeller respectively (m). Ingeneral, the method of the invention may be carried out at an ESI valuefrom about 1 to about 2500, preferably from about 10 to about 250,optimally about 50.

Another important operating parameter of the process of the invention isthe relationship of the impeller diameter and height for a givenvolumetric gas flow rate and impeller tip speed and this parameter maybe termed the Shear Effectiveness Index (SEI). The SEI is determined bythe relationship: ##EQU3## wherein Q is the volumetric gas flow rateinto the impeller (m³ /s), h is the height of the impeller blades (m),D_(i) is the outside diameter of the impeller (m) and V_(i) is theimpeller blade tip velocity (m/s). The present invention generallyemploys an SEI value in the range of about 1 to about 10 and preferablyabout 2 to about 5. These parameters apply to self-induced systemswithout internal baffles as described in U.S. Pat. No. 3,993,563(Degner) and/or to those with externally-sparged gas. The ESI and SEIvalues employed herein preferably are determined for impeller tip speedvelocities of at least about 500 in/sec (at least about 12.5 m/s).

In a further aspect of the invention, there is provided a method for thedistribution of a gaseous phase in a liquid phase, which comprisesproviding a rotary impeller comprising a plurality of blades at asubmerged location in the liquid phase surrounded by a shroud throughwhich are formed a plurality of openings; feeding the gaseous phase tothe submerged location; rotating the impeller about a substantiallyvertical axis at a speed corresponding to a tip speed (V_(i)) of atleast about 500 in/sec. (at least about 12.5 m/s) and in such manner asto establish a Shear Effectiveness Index (SEI) value of about 1 to about10, wherein the SEI value is determined by the relationship: ##EQU4##wherein Q is the gas flow rate into the impeller (m³ /s), h is theheight (m) of the impeller blades, D_(i) is the outside diameter (m) ofthe impeller and V_(i) is the blade tip velocity (m/s), and so as todraw liquid phase into the interior of the shroud and to generatesufficient shear forces between the impeller and the plurality ofopenings in the shroud to distribute the gaseous phase as bubbles in theliquid phase to the interior of the shroud and to effect intimatecontact of the gaseous phase and the liquid phase at the submergedlocation so as to form a gas-liquid mixture of bubbles of the gaseousphase in the liquid phase contained within the shroud while effectingshearing of the gas-liquid mixture within the shroud, flowing thegas-liquid mixture from within interior the shroud through and incontact with the openings to external of the shroud at an EffectiveShear Index (ESI) value of from about 1 to about 2500, wherein the ESIvalue is determined by the relationship: ##EQU5##

The gas velocity index (GVI) is determined by the expression: ##EQU6##where Q is the volumetric flow rate of gas (m³ /s), n is the number ofopenings in the shroud, A is the area of the opening (m²) and P is thelength of the perimeter of the opening (m). V_(i) is the blade tipvelocity (m/s), D, is the inside diameter (m) of the shroud and D_(i) isthe outside diameter (m) of the impeller, so as to effect furthershearing of the gas-liquid mixture and further intimate contact of thegaseous phase and the liquid phase.

The SEI and ESI indices also comprise useful parameters for scale upprocedures and provide a range of feasible parameters. In this regard,for a given volumetric gas flow rate, the SEI value may be used tocalculate impeller size while the ESI value is used to calculate shrouddiameter. Accordingly, an additional aspect of the invention provides amethod for determining the parameters of a gas-liquid contact apparatusfor the removal of a component of a gas stream using animpeller-apertured shroud combination immersed in a liquid phase towhich the gas stream is fed at a volumetric gas flow rate of Q to theimpeller, which comprises quantifying the structural and operatingparameters of the apparatus so as to provide a Shear Effectiveness Index(SEI) value of from about 1 to about 10, wherein the SEI value isdetermined by the relationship: ##EQU7## in which Q is the volumetricgas flow rate to the impeller (m³ /s), h is the height (m) of theimpeller blades, D_(i) is the inside diameter (m) of the impeller andV_(i) is the impeller blade tip velocity (m/s) and so as to provide anEffective Shear Index (ESI) value of from about 1 to about 2500, whereinthe ESI value is determined by the relationship: ##EQU8## in which GVIis the Gas Velocity Index (/s) through each aperture in the shroud,V_(i) is the impeller tip speed velocity (m/s), D, is the insidediameter (m) of the shroud and D_(i) is the outside diameter (m) of theimpeller, where the gas velocity index (GVI) is determined by theexpression: ##EQU9## where Q is the volumetric flow rate of gas (m³ /s),n is the number of openings in the shroud, A is the area of the opening(m²) and P is the length of the perimeter of the opening (m). Theseparameters are particularly useful for determining the structural andoperating parameters of an apparatus to be employed herein at animpeller blade tip velocity of at least about 500 in/sec (at least about12.5 m/s).

By employing the unique combination of impeller blade tip velocity andgas velocity index through the shroud openings as set forth herein,and/or the unique combination of ESI and SEI values as recited above, avery efficient distribution of gas and liquid phases is effected, suchthat rapid and efficient mass transfer occurs. As noted above and asdescribed in detail below, this result may be employed in a variety ofapplications where such rapid and efficient mass transfer, agitation ofthe liquid phase and gas and liquid mixing is desirable and can beeffected in the region of the shroud, as opposed to the body of theliquid medium, as in the case of mineral separation.

Such procedures include:

(a) the removal of gaseous components from gas streams, in particular bychemical conversion of such gaseous components or by physicaldissolution of such gaseous components or by adsorption on a solidphase,

(b). the removal of dissolved components from a liquid phase, inparticular by chemical conversion of the dissolved components by gaseouscomponents of the gas stream or physical desorption of dissolvedcomponents,

(c) the treatment of suspended components in the liquid phase, inparticular by chemical treatment with gaseous components of the gasstream, and

(d) removal of particulates and other non-gaseous components from gasstreams, including thermal energy.

The enhanced efficiency which is achieved in the present inventionresults from high shear rates affecting both the gas and liquid phasesin the shrouded region, whereby an intimate mixture of gaseous andliquid phases is formed confined within the shroud and passage of theintimate mixture through and in contact with the shroud, such as toachieve rapid mass transfer of interactive components one to the other,along with a powerful pumping action of liquid and liquid-gas mixtureprovided by the impeller.

Also as discussed in more detail below, the equipment used in the methodof the present invention, has a superficial similarity to flotationequipment generally employed for the separation of suspended solidcomponents from a liquid phase. However, the present invention employsequipment modifications and operating parameter modifications notemployed in such flotation operations and which contribute to the uniquenature of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an upright sectional view of a novel gas-liquid contactapparatus provided in accordance with one embodiment of the invention;

FIG. 2 is a detailed perspective view of the impeller and shroud of theapparatus of FIG. 1; and

FIG. 3 is a close-up perspective view of a portion of the shroud of FIG.2.

GENERAL DESCRIPTION OF INVENTION

The present invention is directed, in one embodiment, towards improvingthe process of the prior Canadian Patent No. 1,212,819 by modificationto the physical structure of the agitated flotation cell employedtherein and of the operating conditions employed therein, so as toimprove the overall efficiency of hydrogen sulfide removal and therebydecrease operating and capital costs, while, at the same time, retaininga high efficiency for removal of hydrogen sulfide from the gas stream.

However, the present invention is not restricted to effecting theremoval of hydrogen sulfide from gas streams by oxidation, but ratherthe present invention is generally applicable to the removal of gas,liquid and/or solid components from a gas stream by chemical reactionand/or physical process, and more broadly includes the removal ofgaseous phase components in any physical form as well as sensible heatfrom a gas stream by gas-liquid contact, and the removal of componentsfrom a liquid phase, in particular for the chemical conversion ofcomponents from a liquid phase while in contact with the gaseous phase.

In one embodiment of the present invention, a gas stream is brought intocontact with a liquid phase in such a manner that there is efficientcontact of the gas stream with the liquid phase for the purpose ofremoving components from the gas stream, particularly an efficientcontact of gas and liquid is carried out for the purpose of effecting areaction which removes a component of the gas and converts thatcomponent to an insoluble phase while in contact with the liquid phase.However, the removal of a component may be effected by a physicalseparation technique, rather than a chemical reaction. These operationscontrast markedly with the conventional objective of the design of aflotation cell, which is to separate a slurry or suspension into aconcentrate and a gangue or barren stream in minerals beneficiation. Acomponent is not specifically removed from a gas stream during thelatter operations, nor is there an interaction of gaseous phase andliquid phase components. The distribution of the gas phase in the liquidphase in such flotation processes is generally for the sole purpose ofphysical removal of the solid phase by flotation by gaseous bubbles.

There are a variety of processes to which the principles of the presentinvention can be applied. The processes may involve reaction of agaseous component of the gas stream with another gaseous species in aliquid phase, usually an aqueous phase, often an aqueous catalystsystem.

One example of such a process is the oxidative removal of hydrogensulfide from gas streams in contact with an aqueous transition metalchelate system to form sulfur particles, as described generally in theabove-mentioned Canadian Patent No. 1,212,819.

Another example of such a process is in the oxidative removal ofmercaptans from gas streams in contact with a suitable chemical reactionsystem to form immiscible liquid disulfides, potentially in combinationwith hydrogen sulfide removal.

A further example of such a process is the oxidative removal of hydrogensulfide from gas streams using chlorine in contact with an aqueoussodium hydroxide solution, to form sodium sulphate, which, after firstsaturating the solution, precipitates from the aqueous phase.

An additional example of such a process is the removal of sulfur dioxidefrom gas streams by the so-called "Wackenroder's" reaction by contactinghydrogen sulfide with an aqueous phase in which the sulfur dioxide isinitially absorbed, to form sulfur particles. This process is describedin U.S. Pat. Nos. 3,911,093 and 4,442,083. The procedure of the presentinvention also may be employed to effect the removal of sulfur dioxidefrom a gas stream into an absorbing medium in an additional gas-liquidcontact vessel.

A further example of such a process is the removal of sulfur dioxidefrom gas streams by reaction with an aqueous alkaline material.

The term "insoluble phase" as used herein, therefore, encompasses asolid insoluble phase, an immiscible liquid phase and a component whichbecomes insoluble when reaching its solubility limit in the liquidmedium after start up.

The component removed from the gas stream in this embodiment of theinvention usually is a gaseous component but the present inventionincludes the removal of other components from the gas stream, such asparticulate material or dispersed liquid droplets.

For example, the present invention may be employed to remove solidparticles, such as sulfur dust, or liquid droplets from a gas stream,for example, aerosol droplets, such as sulfuric acid mist, such as byscrubbing with a suitable liquid medium. Any component lacking anaffinity for the liquid phase removed from a gas stream in such aprocedure may be removed therefrom, such as by flotation, while anycomponent having an affinity for the liquid phase removed from a gasstream in this way may-remain therein. A foam formed on the surface ofthe liquid phase may trap floated particulate matter and/or residualaerosol material. Similarly, moisture may be removed from a gas stream,such as by scrubbing with a suitable hydrophilic organic liquid, such asglycol.

A wide range of particle sizes from near molecular size through Aitkennuclei to visible may be removed from a gas stream by the wellunderstood mechanisms of diffusion, interception, impaction and capturein a foam layer using the method described herein.

More than one component of any type and components of two or more typesmay be removed simultaneously or sequentially from the gas stream. Inaddition, a single component may be removed in two or more sequentialoperations.

The present invention also may be employed to remove sensible heat (orthermal energy) from a gas stream by contacting the gas stream with asuitable liquid phase of lower temperature to effect heat exchange.Similarly, sensible heat may be removed by evaporation of a liquidphase. In addition, a hot liquid medium may be contacted by a coolgaseous medium.

Accordingly, in one preferred aspect of the present invention, there isprovided a method of removing a component from a gas stream containingthe same in a liquid phase, comprising a plurality of steps. Acomponent-containing gas stream is fed to a gas-liquid contact zone inwhich is located a liquid medium, and which may be enclosed.

An impeller comprising a plurality of blades is rotated about agenerally vertical axis at a submerged location in the liquid medium,while the gas stream is flowed along a generally vertical flow path fromexternal to the gas-liquid contact zone to the submerged location.

The impeller is surrounded by a shroud through which are formed aplurality of openings, generally within a preferred range of impeller toshroud diameter ratios as determined by the ESI index as discussedabove. The impeller is rotated at a speed corresponding to a blade tipvelocity of at least about 350 in/sec. (9 m/s), preferably at leastabout 500 (12.5 m/s), for example, up to or greater than about 700in/sec. (18 m/s), so as to generate sufficient shear forces between theimpeller blades and the plurality of openings in the shroud todistribute the gas stream as gas bubbles in the liquid medium to theinterior of the shroud, thereby achieving intimate contact of thecomponent and liquid medium at the submerged location so as to form agas-liquid mixture of gas bubbles in the liquid medium contained withinthe shroud while effecting shearing of the gas-liquid mixture within theshroud and to effect removal of the component from the gas stream intothe liquid medium. The impeller may be constructed and rotated in such amanner as to provide an SEI value, as defined above, of about 1 to about10, preferably about 4 to about 5.

The gas-liquid mixture from within the interior of the shroud throughand in contact with the openings therein into the body of the liquidmedium external to the shroud at a gas velocity index (GVI) at leastabout 18 per second per opening, preferably at least about 24 per secondper opening, so as to effect further shearing of the gas-liquid mixtureand further intimate contact of the gas stream and the liquid medium,whereby any removal of component not effected in the interior of theshroud is completed in the region of the liquid medium adjacent to theexterior of the shroud.

In this embodiment, as well as the other embodiments of the invention,the gas velocity index (GVI ) more preferably is at least about 30 persecond per opening, and may range to very high values, such as up toabout 500 per second per opening, and often is in excess of about 100per second per opening. The impeller and shroud arrangement may providean ESI value, as defined above, of about 1 to about 2500, preferablyabout 10 to about 250.

As mentioned above, the gas velocity index (GVI) is determined by theexpression: ##EQU10## where Q is the volumetric flow rate of gas (m³/s), n is the number of openings in the shroud, A is the area of theopening (m²) and P is the length of the perimeter of the opening (m ).

A component-depleted gas stream is vented from a gas atmosphere abovethe liquid level in the gas-liquid contact zone.

While the gas-liquid contact procedure is generally operated with anenclosed reaction zone operating at or near atmospheric pressure, italso is possible to carry out the gas stream component removal methodunder superatmospheric and subatmospheric conditions, depending oncircumstances and requirements.

While the present invention, in the gas stream component removalembodiment, is described specifically with respect to the removal ofhydrogen sulfide and sulfur dioxide from gas streams containing the sameby reaction to form sulfur and recovery of the so-formed sulfur byflotation by bubbles of the component-depleted gas stream, it will beapparent from the foregoing and subsequent discussion that both theapparatus provided in accordance with an aspect of the present inventionand the gas stream component removal method embodiment of the inventionare useful for effecting other procedures where a component of a gasstream is removed in a liquid medium or a component of a liquid mediumis removed or treated by contact with a gaseous component. In addition,it will be apparent that the present invention broadly relates to methodand apparatus for contacting a gas phase with a liquid phase for avariety of purposes.

In one preferred aspect of the invention, hydrogen sulfide contained ina gas stream is converted to solid sulfur particles by oxygen in anaqueous transition metal chelate solution as a reaction medium. Theoxygen employed in this conversion process is present in anoxygen-containing gas stream which is introduced to the same submergedlocation in the aqueous catalyst solution as the hydrogensulfide-containing gas stream, either in admixture therewith or as aseparate gas stream. The oxygen-containing gas stream similarly issheared by the rotating impeller-shroud combination, which achievesintimate contact of oxygen and hydrogen sulfide with each other and theaqueous catalyst solution to effect the oxidation. The hydrogen sulfideis removed by chemical conversion to insoluble sulfur particles.

The solid sulfur particles may be permitted to grow or are subjected tospherical agglomeration or flocculation until they are of a size whichenables them to be floated from the body of the reaction medium to thesurface thereof by hydrogen sulfide-depleted gas bubbles.

The sulfur is generally of crystalline form and particles of sulfur aretransported by the hydrogen-sulfide depleted gas bubbles from thereaction medium to the surface thereof when having a particle size offrom about 10 to about 50 microns in diameter to form a sulfur frothfloating on the surface of the aqueous medium and a hydrogensulfide-depleted gas atmosphere above the froth, from which is vented ahydrogen sulfide-depleted gas stream. The sulfur-bearing froth may beremoved from the surface of the aqueous medium to exterior of theenclosed reaction zone, either on a continuous or intermittent basis.

Sulfur formed in such hydrogen sulfide-removing processes has been foundto be highly adsorbent of other odiferous components, such as odiferoussulfurous and/or nitrogenous compounds, and hence which are removed onsulfur formed during the hydrogen sulfide oxidation. This result makesthe process particularly useful in the treatment of exhaust gas streamsfrom meat rendering plants, pulp mills and gas process plants whichcontain a large variety of odiferous sulfur and nitrogen compounds, inaddition to hydrogen sulfide, which are adsorbed by the sulfur and henceare removed from the gas stream, thereby permitting an odour-reduced gasstream to be vented from the plant. The use of freshly precipitated highsurface area sulfur for the removal of odiferous gases from gas streamsis an additional aspect of the present invention.

Accordingly, in an additional aspect of the present invention, there isprovided a continuous method for the removal of components from a gasstream comprising a component oxidizable to sulfur in a liquidcatalyst-containing medium and odiferous components, which comprisescontinuously forming sulfur in a liquid phase from the componentoxidizable to sulfur continuously adsorbing odiferous components fromthe gas stream on the continuously-formed sulfur, and continuouslyremoving sulfur from the liquid phase.

Since sulfur is formed continuously from the hydrogen sulfide or othersulfur-forming component and floated from the liquid phase, the sulfurparticles in the froth on the surface of the liquid are continuouslyremoved, so that the odiferous compounds are continuously removed fromthe gas phase.

High levels of hydrogen sulfide removal efficiency may be attained usingthe method of the present invention, if desired, in excess of 99.99%,from gas streams containing any concentration of hydrogen sulfide.Residual concentrations of hydrogen sulfide less than 0.1 ppm by volumecan be attained, if desired. Corresponding removal efficiencies areachieved for the removal of other gaseous components from gas streams.

The method of the invention is able to remove effectively hydrogensulfide from a variety of different source gas streams containing thesame, provided there is sufficient oxygen present and dispersed in thereaction medium to oxidize the hydrogen sulfide. The oxygen may bepresent in the hydrogen sulfide-containing gas stream to be treated ormay be separately fed, as is desirable where natural gas or othercombustible gas streams are treated.

Hydrogen sulfide containing gas streams which may be processed inaccordance with the invention include fuel gas, anaerobic digester gas,and natural gas and other hydrogen sulfide-containing streams, such asthose formed in oil processing, oil refineries, mineral wool plants,kraft pulp mills, rayon manufacturing, heavy oil and tar sandsprocessing, coal coking, meat rendering, a foul gas stream produced inthe manufacture of carborundum and gas streams formed by air strippinghydrogen sulfide from aqueous phases. The gas stream may be onecontaining solids particulates or may be one from which particulates areabsent. The ability to handle a particulate-laden gas stream in thepresent invention without plugging may be beneficial, since thenecessity for upstream cleaning of the gas is obviated.

The method of the present invention as it is applied to effectingremoval of hydrogen sulfide from a gas stream containing the samegenerally employs a transition metal chelate in aqueous medium as thecatalyst for the oxidation of hydrogen sulfide to sulfur. The transitionmetal usually is iron, although other transition metals, such asvanadium, chromium, manganese, nickel and cobalt may be employed,provided that such metals have multiple valence states -and possess therequired oxidation potential. Any desired chelating agent may be usedprovided that it possesses the appropriate complex formation constant,but generally, the chelating agent is ethylenediaminetetraacetic acid(EDTA). An alternative chelating agent is HEDTA. The transition metalchelate catalyst may be employed in hydrogen or salt form. The

operative range of pH for the process generally is about 7 to about 11.The transition metal chelate may be added as such or may be formed insitu.

At elevated temperatures of operation, the complex iron chelate catalysttends to break down and form suspension of iron hydroxide in the aqueousmedium. Surprisingly, the conversion of hydrogen sulfide sulfur remainseffective in the presence of such precipitated iron hydroxide. Thisresult is particularly surprising since prior attempts have been made touse iron hydroxide with only a limited degree of success, as a result offormation of ferrous sulfide due to the absence of mass transfer ratesof oxygen sufficient to oxidize the ferrous sulfide. The iron hydroxideparticles are very finely divided and well dispersed in the aqueousphase and are maintained in suspension by the circulation of liquid inthe reaction medium provided by the impeller. In place of iron hydroxideas the catalytic component, there may be employed another transitionmetal hydroxide.

One effect of the utilization of such finely divided ferric hydroxidefor the oxidative conversion of hydrogen sulfide to sulfur is that thesulfur is obtained in a very finely-divided, free-flowing form, and canbe quite different from the physical form of the sulfur obtained usingiron chelate.

The microcrystalline sulfur which is obtained is free-flowing and has aparticle size distribution of less than about 10 microns and an averageparticle size of about 1 micron or less and is a useful commercialcommodity, as a pesticide or fungicide. The microcrystalline sulfur,which forms one embodiment of the invention, generally has free ironhydroxide adsorbed thereon from the catalyst in the reaction medium,which is beneficial in some uses of the sulfur.

The iron hydroxide used in the catalytic conversion of hydrogen sulfideto sulfur is provided preferably as freshly precipitated material formedfrom iron chelate solution or other convenient source, such as reactionof an iron salt, such as ferric chloride or ferric sulfate, with sodiumhydroxide, since the iron hydroxide is thereby provided a very finelydivided form. However, iron hydroxide from commercial sources also maybe employed. Where the sulfur produced is contaminated by ironhydroxide, the sulfur may be treated with a solution of EDTA or othercomplexing agent, or with acid to remove the iron contaminant, and thenwashed and dried.

The hydrogen sulfide removal process of the invention is convenientlycarried out at ambient temperatures of about 20° to 35° C., althoughhigher and lower temperatures may be adopted and still achieve efficientoperation. The temperature generally ranges from about 5° to about 80°C., although higher temperature may be employed.

The minimum catalyst concentration to hydrogen sulfide concentrationratio for a given gas throughput may be determined from the rates of thevarious reactions and of mass transfer occurring in the process and isinfluenced by the temperature and the degree of agitation or turbulencein the reaction vessel. This minimum value may be determined for a givenset of operating conditions by decreasing the catalyst concentrationuntil-the removal efficiency with respect to hydrogen sulfide begins todrop sharply. Any concentration of catalyst above this minimum may beused, up to the catalyst loading limit of the system.

The removal of hydrogen sulfide by the process of the present inventionis carried out in an enclosed gas-liquid contact zone in which islocated an aqueous medium containing transition metal chelate catalyst.A hydrogen sulfide-containing gas stream and an oxygen-containing gasstream, which usually is air but may be pure oxygen or oxygen-enrichedair, may be caused to flow, either separately or as a mixture, along avertical flow path from outside the gas-liquid contact zone to asubmerged location in the aqueous catalyst medium, or otherwiseintroduced to the submerged location, from which the mixture is forcedby the rotating impeller to flow through the shroud openings into thebody of the aqueous medium. The rotating impeller also draws the liquidphase from the body of aqueous medium in the enclosed zone to thelocation of introduction of the gas streams, interior of the shroud.

As described above, the gas streams are distributed as bubbles by thecombined action of the rotating impeller and the surrounding shroudwhich has a plurality of openings therethrough. To achieve goodgas-liquid contact and hence efficient oxidation of hydrogen sulfide tosulfur, the impeller is rotated rapidly so as to achieve a blade tipvelocity of at least about 350 in/sec (9 m/s), preferably at least about500 (12.5 m/s), for example, up to and above about 700 in/sec. (18 m/s).In addition, shear forces between the impeller and the stationary shroudassist in achieving the good gas-liquid contact by providing a gasvelocity index (as defined above) which is at least about 18 per secondper opening, preferably at least about 24 per second per opening. Inthis aspect of the invention and the others described herein, other thanat or near the upper limit of capacity of a unit, the gas flow ratethrough the openings, measured at atmospheric pressure, is less thanabout 0.02 lb/min/opening in the shroud, generally down to about 0.004,and preferably in the range of about 0.005 to about 0.007lb/rain/opening in the shroud. In addition, the procedure may beoperated at an ESI value of about 1 to about 2500, preferably about 10to about 250, optimally at about 50, and an SEI value of about 1 toabout 10, preferably about 4 to about 5.

The high shear rate of the gas-liquid mixture provided by theimpeller-shroud combination used herein enables a high rate of masstransfer to occur. In the catalyst solution, a complicated series ofchemical reactions occurs resulting in an overall reaction which isrepresented by the equation:

    H.sub.2 S+1/2 O.sub.2 →S+H.sub.2 O.

The overall reaction thus is oxidation of hydrogen sulfide to sulfur.

As noted earlier, the solid sulfur particles may be allowed to grow insize until of a size which can be floated. Alternative procedures ofincreasing the particle size may be employed, including sphericalagglomeration or flocculation. The flotable sulfur particles are floatedby the hydrogen sulfide-depleted gas bubbles rising through the body ofcatalyst solution and collected as a froth on the surface of the aqueousmedium. The sulfur particles range in size from about 10 to about 50microns in diameter and are generally in crystalline form.

The series of reactions which is considered to occur in the metalchelate solution to achieve the overall reaction noted above is asfollows:

H₂ S =H⁺ +HS-

OH-+FeEDTA-=[Fe.OH.EDTA]=

HS-+[Fe.OH.EDTA]=[Fe.HS.EDTA]=+OH-

[Fe.HS.EDTA]==FeEDTA-+S+H⁺ +2e

2e +1/2 O₂ +H₂ O=20H-

2H⁺ +20H=2H₂ O

Alternatively, the oxygen-containing gas stream may be introduced to themetal chelate solution at a different submerged location from thehydrogen sulfide-containing gas stream using a second impeller/shroudcombination, as described in more detail in U.S. Pat. No. 5,403,567,assigned to the assignee hereof, the disclosure of which is incorporatedherein by reference;

In another preferred aspect of the present invention, sulfur dioxide isreacted with an alkaline medium to remove the sulfur dioxide from a gasstream bearing the same. Sulfur dioxide is absorbed from the gas streaminto the aqueous alkaline medium and reacts with active alkali thereinto form salts, with the sulfur dioxide-depleted gas stream being ventedfrom the reaction medium. The procedure shows many similarities with thehydrogen sulfide-removal procedure just described, except that theaqueous medium contains a non-catalytic alkaline material.

The aqueous alkaline medium into which the sulfur dioxide-containing gasstream is introduced may be provided by any convenient alkaline materialin aqueous dissolution or suspension. One convenient alkaline materialwhich can be used is an alkali metal hydroxide, usually sodiumhydroxide. Another convenient material is an alkaline earth metalhydroxide or carbonate, usually a lime slurry or a limestone slurry.

Absorption of sulfur dioxide in an aqueous alkaline medium tends toproduce the corresponding sulfite. It is preferred, however, that thereaction product be the corresponding sulfate, in view of the greatereconomic attraction of the sulfate salts. For example, where lime orlimestone slurry is used, the by-product is calcium sulfate (gypsum), amulti-use chemical.

Accordingly, in a preferred aspect of the invention, anoxygen-containing gas stream, which usually is air but which may be pureoxygen, oxygen-enriched air or ozone, analogously to the case ofhydrogen sulfide, also is introduced to the aqueous alkaline reactionmedium, so as to cause the sulfate salt to be formed. When suchoxidation reaction is effected in the presence of a lime or limestoneslurry, it is generally preferred to add a small amount of ananti-caking agent, to prevent caking of the by-product calcium sulfateon the lime or limestone particles, decreasing their effectiveness. Onesuitable anti-caking agent is magnesium sulfate.

The concentration of sulfate salt builds up in the aqueous solutionafter initial start up until saturates the solution, whereupon thesulfate commences to precipitate from the solution.

The oxygen-containing gas stream, when used, may be introduced to theaqueous medium at the same submerged location as the sulfurdioxide-containing gas stream, either in admixture with the sulfurdioxide-containing gas stream or as a separate gas stream.

Alternatively, the oxygen-containing gas stream may be introduced to theaqueous alkaline medium at a different submerged location from thesulfur dioxide-containing gas stream using a second impeller/shroudcombination, as described in more detail in the aforementioned U.S. Pat.No. 5,403,567.

The process of the invention is capable of rapidly and efficientlyremoving sulfur dioxide from gas streams containing the same. Such gasstreams may contain any concentration of sulfur dioxide and the processis capable of removing such sulfur dioxide in efficiencies exceeding99.99%. Residual sulfur dioxide concentrations below 0.1 ppm by volumecan be achieved.

This sulfur dioxide removal embodiment of the invention can be-carriedout under a variety of process conditions, the choice of conditionsdepending, to some extent, on the chemical imparting alkalinity to thereaction medium. For an alkali metal hydroxide, the aqueous alkalinesolution generally has a concentration of from about 50 to about 500g/L. For an alkaline earth metal hydroxide or carbonate, the aqueousalkaline solution-generally has a concentration of from about 1 to about20wt %. The active alkalinating agent may be continuously andintermittently replenished to make up for the conversion to thecorresponding sulfite or sulfate. The reaction temperature may varywidely from about 5° to about 100° C. or higher, if a superatmosphericpressure is applied to the liquid phase.

One specific use to which the present invention may be put is in thedesulfurization of gas streams containing sulfur dioxide and hydrogensulfide by reacting the gases together by the Claus reaction in a liquidenvironment. One such application of such procedures is in the treatmentof tail gas from a Claus reactor used in the natural gas refiningindustry.

In a Claus reactor, hydrogen sulfide and sulfur dioxide react togetherat high temperature greater than about 600° C. over a fixed catalyticbed, usually of bauxite. The sulfur dioxide for such reaction usually isformed by oxidation of a portion of the hydrogen sulfide contained in afeed stream. The process is over efficient in removing the hydrogensulfide and producing liquid sulfur. The tail gas from such operation,however, contains low concentrations of H₂ S and SO₂ which yet aresufficiently high as to preclude environmental discharge and a tail gasclean-up unit often is required.

The tail gas stream is processed in one embodiment of the presentinvention to remove substantially all hydrogen sulfide and sulfurdioxide from the gas stream, to enable the purified gas stream to bedischarged to the environment. The procedure involves a two-stepoperation, in which substantially all the sulfur dioxide first isremoved by reaction with hydrogen sulfide in the tail gas stream in aliquid medium, which may be an aqueous medium, in a first reactionvessel by using the impeller and shroud combination provided herein todistribute the gases in the liquid phase. The combination of impellerand shroud provides a highly efficient contact of the gases with eachother and the liquid phase, to cause reaction of sulfur dioxide andhydrogen sulfide to sulfur. The liquid phase may contain a suitablecatalyst material for the reaction, as described in U.S. Pat. No.4,442,083.

The concentration of sulfur dioxide, which preferably is less thanstoichiometric, is rapidly and effectively removed from the tail gasstream by this procedure, along with a proportion of hydrogen sulfidereacting with the sulfur dioxide. By-product sulfur from this reactionmay be floated or otherwise removed from the liquid phase, as describedin more detail elsewhere herein.

The rate of conversion of sulfur dioxide and hydrogen sulfide to sulfurin a liquid phase may be and generally is dependent on the rate ofabsorption of hydrogen sulfide and sulfur dioxide. The impeller shroudcombination employed herein providing a high degree of shearing of thegases and agitation of the liquid and gaseous phases effects masstransfer of the gases in a very effective manner. Once distributed intothe liquid phase, the gases form species which can interact and react toform sulfur..

The various reactions involved in an aqueous system may be depicted asfollows:

(a) Absorption:

2H₂ s(g)→2H₂ S(aq)

SO₂ (g)→SO₂ (aq)

(b) Ionization:

2H₂ S(aq)→2HS-+2H⁺

SO₂ (aq)+H₂ O→HSO₃ -+H⁺

(c) Oxidation/Reduction:

HSO₃ ⁻ +4e-+5H⁺ →S+3H₂ O

2HS-→2S+2H⁺ +4e

(d) overall:

2H₂ S+SO₂ →3S+2H₂ O

The ionization step is facilitated by the employment of an aqueousphase. The solubility of HS- increases rapidly with increasing pH to amaximum level at about pH 8 while the solubility of HSO₃ ⁻ increasesrapidly to a maximum level at about pH 2 and declines rapidly beyond apH of about 7. The maximum concentrations of HS- and HSO₃ ⁻ occur atabout pH 6.5. However, prior studies have suggested that a pH in therange of about 3 to about 5.5 are most effective for producing sulfur. Arange of pH of about 1.5 to about 8.5 may be employed. A phosphatebuffer, such as a mixture of phosphoric acid and potassium dihydrogenphosphate, may be added to maintain a constant pH during the reaction,as suggested in U.S. Pat. No. 3,911,093, and to promote the ionizationreactions. Other liquid media, such as tricresyl phosphate and liquidsulfur may be used to effect such processes.

While room temperature and atmospheric pressure conditions may beemployed effectively for the liquid phase reaction, it is possible touse a wide range of temperature for the reaction, for example, fromabout 5° C. to about 80° C., and higher, for an aqueous system.

The sulfur dioxide-depleted tail gas stream containing residual amountsof hydrogen sulfide then may be forwarded to a second reaction vesselalso containing an impeller-shroud combination as described herein,along with an aqueous catalytic medium in which the hydrogen sulfide isoxidized to sulfur, as described above. Any mercaptans and other organicsulfides hat may be present are adsorbed on the sulfur as describedpreviously. The clean gas stream is vented from the second reactor. Theremoval of sulfur dioxides and the organic sulfides in this manneravoids the necessity to employ a hydrotreater, an expensive piece ofequipment.

The latter procedure may be employed on gas streams comprising hydrogensulfide, wherein a portion of the hydrogen sulfide is initially oxidizedto sulfur dioxide, and the resulting stream then is subjected to theliquid Claus reaction to remove substantially the sulfur dioxide andthen subject the residual stream to a hydrogen sulfide removal process.

Such a two-step treatment process is a novel procedure applicable to theremoval of hydrogen sulfide from gas streams independent of the specificmeans of gas-liquid contacting employed in each of the process steps foran overall net cost saving in chemicals. Accordingly, in a furtheraspect of the present invention, there is provided a process for theremoval of hydrogen sulfide from a gas stream containing the same, whichcomprises oxidizing a portion of the hydrogen sulfide therein to formsulfur dioxide therefrom and to produce an oxidized gas streamcontaining a stoichiometric excess of hydrogen sulfide; reactingsubstantially all the sulfur dioxide in the oxidized gas stream withhydrogen sulfide remaining in the oxidized gas stream in a liquid phaseto form sulfurous material therefrom contained in the liquid phase andto produce a further gas stream having a decreased hydrogen sulfidecontent; oxidizing substantially all hydrogen sulfide remaining in thefurther gas stream in a liquid phase to form a sulfurous materialtherefrom; and venting a gas stream substantially free from gaseoussulfur compounds. By adopting such a procedure, the quantity ofchelating agent required to effect hydrogen sulfide oxidation issignificantly decreased, compared to a procedure in which the initialgas stream is directly treated by an aqueous catalyst system.

Another application of the process of the invention is the carbonatingof lime suspension to produce precipitated calcium carbonate, byutilizing the impeller-shroud combination to introduce carbon dioxideinto a suspension of lime in a reactor. The lime is maintained insuspension by the agitation and circulation produced by theimpeller-shroud combination. Finely-divided calcium carbonate (whiting)is produced by this procedure and is a high value-added product, havingutility in the paper coating and plastics industries, among others.

The process of the present invention further may be employed to effectselective removal of sulfur dioxide from gas streams containing thesame, for example, carbon dioxide-containing gas stream intended for usein the carbonating of lime as described above and contaminated withsulfur dioxide. For many applications, finely-divided calcium carbonateis required having less than 0.2 wt % of any insoluble component. Forinitial removal of sulfur dioxide, a slurry of pulverized calciumcarbonate may be employed, as described elsewhere herein.

Such pulverized calcium carbonate or limestone also may be employed inthe process of the invention to effect pH maintenance in systemsscrubbing acid gases, such as hydrogen sulfide, sulfur dioxide ormixtures thereof. For example, with a mixture of hydrogen sulfide andsulfur dioxide, hydrogen sulfide may be removed by oxidation to sulfur,such as in an iron chelate catalyst solution, as described elsewhereherein, while the sulfur dioxide is scrubbed from the gas stream by thecalcium carbonate.

Elemental sulfur produced by such procedure may be preferentiallyseparated from the produced calcium carbonate suspension by flotation.However, the limestone slurry, which eventually becomes depleted ascalcium sulfate, or gypsum plus sulfur, may be added to incinerationash, or used in agriculture as a calciumsulfur supplement, for example,as a soil conditioner, or may be disposed of as land-fill.

In addition to the removal of gaseous components from a gas stream asparticularly described above, the procedure of the present invention,employing the impeller-shroud combination, and the operating parametersof impeller tip speed velocity and gas velocity index through theshroud, and/or ESI and SEI values, also may be used in other instanceswhere distribution of gas phase in a liquid phase is desired andintimate contact of gaseous and liquid phases is desired.

For example, the procedure of the invention may be employed in wastewater treatment, where undesired dissolved components in the liquidphase, including both BOD and COD, are removed by oxidation by oxygen,ozone or chlorine dioxide contained in a gas stream and subjected tohigh shear in the manner described above. Alternatively, hydrogensulfide may be removed from liquid media, for example, hydrogen sulfidemay be stripped from sour water or may be stripped from liquid sulfur,as described in more detail below.

An application of the process of the invention in the pulp and paperindustry is the oxidation and/or stripping of components of white, greenor black liquor. White liquor is a solution of sodium sulfide and sodiumhydroxide used to form wood pulp from wood chips. Oxidation of suchmaterial may be achieved by dispersing an oxidizing gas stream in thepulp liquor using the procedures described herein.

The dispersion of gaseous phase in the liquid phase in the presentinvention may be combined with other components to effect the desiredreaction or interaction between gaseous and liquid phase components. Asdescribed above, such additional components may comprise a catalystdissolved in the liquid phase promoting reaction between gaseouscomponents.

Another example is the removal of the volatile organic compounds(VOC's), as well as semi-volatile organic compounds, from aqueousstreams using a gas subjected to high shear rates in an impeller andshroud combination and the process conditions described herein.

In this procedure, a multi-stage unit may be employed consisting ofthree or four independent impeller systems, together with a feed ofoxygen or other oxygen-containing gas, such as air, possibly underpressure. The water to be treated to remove VOCs then may be introducedat one end of the series of contactors and removed, after treatment, atthe other end. In each contacting stage, the VOCs are stripped from theaqueous phase. Concentrated VOCs then may be passed in contact with acatalyst for oxidation of VOCs, for example, by oxygen or ozone tocarbon dioxide and similar oxidation products external to the strippingoperation. Alternatively, the VOCs may be recovered.

If desired, liquid circulation within the contactor can be controlled bya series of overflow-underflow. weirs, ensuring good gas-liquid contactand a reasonable residence time of liquid within each contact cell. Theliquid flow rate can be modified over a wide range depending on thecontaminant level and activity.

In some instances of utilization of the procedure described herein, itis desirable to operate with the interior of the gas-liquid contactorunder pressure, for example, where temperatures over 100° C. are usedwith aqueous systems.

A problem arises, however, in ensuring an adequate seal between arotating drive shaft for the impeller passing through an upper closureto the gas-liquid contactor to a device motor. This difficulty may beovercome by employing a hydraulically- or pneumatically-driven motorlocated in the interior of the gas-liquid contact vessel with tubescarrying fluid passing through static seals from the exterior of the gasliquid contact vessel to the motor. Such static seals are much easier tomaintain than the dynamic seals required for drive shafts.

In addition, the use of a hydraulically- or pneumatically-driven motorin place of an electrically-driven motor provides a safety factor for adrive motor located within the enclosure, particularly if combustiblegases are present which might be ignited by an electric spark.

Another application of the impeller-shroud combination provided hereininvolves the processing of viscous liquids, such as liquid sulfur andasphalt. Such processing may involve simple circulation of the viscousliquid in a vessel, stripping of absorbed gases from the viscous liquidand/or transfer of gaseous components into the viscous liquid. Theparameters of operation and the physical structure of theshroud-impeller combination used in those latter procedures may be butnot necessarily be different from those employed with aqueous and otherlow viscosity media as described elsewhere herein.

Accordingly, an additional aspect of the present invention provides amethod for the processing of a viscous liquid, which comprises immersinga rotary impeller comprising a plurality of blades in a mass of theviscous liquid surrounded by a shroud through which are formed aplurality of openings, and rotating the impeller about a substantiallyvertical axis at a speed sufficient to draw liquid phase from the massand to flow the liquid phase through the openings in the shroud to causecirculation-of the mass of viscous liquid, along with a desorbing of anabsorbed component from the viscous liquid, and/or transfer of a gaseouscomponent to the viscous liquid.

Liquid sulfur is a by-product from the removal of hydrogen sulfide fromoil refining or sour gaseous streams by the Claus process, whereinhydrogen sulfide is reacted with sulfur dioxide. Hydrogen sulfide issoluble to a limited degree in the liquid sulfur and small quantities ofunconverted hydrogen sulfide, usually about 50 to 300 ppmw, becomedissolved in the liquid sulfur during the course of the Claus process.Such dissolved hydrogen sulfide is reactive with the sulfur and tends toproduce and reach an equilibrium with hydrogen polysulfides.

Hydrogen sulfide concentrations tend to build up in the head space oftank cars used to ship the liquid sulfur, as a result of slow diffusionof the hydrogen sulfide produced from the decomposition of hydrogenpolysulfides contained in the liquid sulfur, often to lethal orexplosive levels. During processing of the liquid sulfur, a largefraction of the hydrogen sulfide is stripped, creating handlingproblems. Some hydrogen sulfide as well as hydrogen polysulfides remainentrapped in the solid sulfur product. The hydrogen polysulfides tend tobreak down over time to sulfur and hydrogen sulfide. As a result, theodour of hydrogen sulfide can be detected at storage sites, duringtransportation and even several weeks after a customer has received theproduct, as the hydrogen sulfide slowly diffuses out of the solidsulfur.

Several proposals have been made to degas liquid sulfur to removeddissolved hydrogen sulfide, but these processes are either costly and/orineffective.

In accordance with the present invention, a shrouded-impeller may beemployed to effect circulation of liquid sulfur to assist indegassification of the liquid sulfur by convection as a result ofagitation of the liquid sulfur by the impeller and shroud, with thehydrogen sulfide-containing product gas stream being further processed,such as described above by oxidative conversion to sulfur in an aqueouscatalyst system.

In addition, the shroud-impeller may be employed to introduce anoxidizing gas, for example, air or oxygen, to the liquid sulfur toeffect a combination of operations to remove hydrogen sulfide andhydrogen polysulfides, by stripping of hydrogen sulfide, oxidation ofhydrogen sulfide to sulfur and oxidation of hydrogen polysulfides tosulfur with or without the use of reagents, as a result of the highlevels of mass transfer of an oxidizing gas, such as oxygen or sulfurdioxide, and agitation achieved using the shroud-impeller combination.Preferably, the liquid sulfur is saturated with oxygen or otheroxidizing gas by such distribution to ensure complete removal ofhydrogen sulfide and hydrogen polysulfides from the liquid sulfur. Byremoving hydrogen sulfide and hydrogen polysulfides from the liquidsulfur in this way, the prior art handling problems with respect toliquid sulfur and solidified sulfur produced therefrom are overcome.

As noted above, another viscous liquid which may be processed using animpeller-shroud combination is liquid asphalt. Concern has beenexpressed that some of the volatile compounds which are released fromasphalt may be harmful to human health, particularly during pavingactivities when large surface areas are exposed to the atmosphere. Inaddition, it is become more common to modify the properties of asphaltby the addition of polymeric materials such as plastomers andelastomers, including materials derived from automotive tires. 0noccasion, in such processes, sulfur may be added to the asphalt toincrease crosslinking, and hydrogen sulfide may be released to theatmosphere.

The addition of oxygen to asphalt tends to create a product which isharder or increased in viscosity, but also tends to diminish theasphalts anti-aging properties, and hence this practice has generallybeen restricted to asphalt used in the production of roofing materials.

Oxidation of the liquid asphalt may be avoided by the use of an inertgas, such as nitrogen or carbon dioxide, to strip odiferous and/orvolatile components from the asphalt, thereby decreasing the evolutionof such components during paving operation and/or to improve itsproperties. An additional source of inert gas may be the exhaust from acombustion engine which may be used to strip the undesired componentsfrom the asphalt.

In addition, asphalt may be treated with an oxidizing gas, such asoxygen, oxygen enriched air or air, to alter the properties of theasphalt, for example, to harden it.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings, a novel gas-liquid contact apparatus 10,provided in accordance with one embodiment of the invention, is amodified form of an agitated flotation cell. The design of thegas-liquid contactor 10 is intended to serve the purpose of efficientlycontacting gases and liquids, for example, to effect removal of acomponent of the gas, such as by reaction to produce a flotableinsoluble phase, but also applicable to the chemical conversion ofaqueous phase components by gaseous phase components dispersed in theliquid phase. This design differs from that of an agitated flotationcell whose objective is to separate a slurry or suspension into aconcentrate and a gangue or barren stream. Where such apparatus isintended to be employed for treatment of liquid phase material,appropriate inlet and outlet posts are provided.

The reactor 10, constructed in accordance with one embodiment of theinvention and useful in chemical and physical processes for removing acomponent from a gas stream, such as oxidative removal of hydrogensulfide, and other gas-liquid contacting processes, such as describedabove, comprises an enclosed housing 12 having a standpipe 14 extendingfrom exterior to the upper wall 16 of the housing 12 downwardly into thehousing 12. The housing 12 may be of any convenient shape, generallycircular. The housing 12 may be designed such as to avoid dead zones inthe liquid phase contained within the housing.

Inlet pipes 18,20 communicate with the standpipe 14 through an inletmanifold at its upper end for feeding gas streams, in this illustratedembodiment, hydrogen sulfide-containing gas stream and oxygen-containinggas stream, (air) to reactor 10. The inlet pipes 18,20 have inletopenings 22,24 through which the gas flows. The openings are designed toprovide a low pressure drop.

Generally, the flow rate of gas streams may range upwardly from aminimum of about 50 cu.ft/min. (about 25 dm³ /s), for example, in excessof about 5000 cu.ft/min. (about 2500 dm³ /s), although much higher orlower flow rates may be employed, depending on the intended applicationof the process. The pressure drop across the unit may be quite low andmay vary from about -5 to about +20 in. H₂ O (from about -250 to about+500 mm H₂ O) , preferably from about 0 to less than about 10 in. H₂ O(-250 mm H₂ O). For larger units employing a fan or a blower to assistthe gas flow rate to the impeller, the pressure drop may be greater.

A shaft 26 extends into the vessel 10 and has an impeller 28 mounted atits lower end just below the lower extremity of the standpipe 14. Adrive motor 30 is mounted to drive the shaft 26. Although there isillustrated in the drawings an apparatus 10 with a single impeller 28,it is possible to provide more than one impeller and hence more than oneoxidative reaction (or other chemical or physical process) location inthe same enclosed tank. The gas flow rate to the reactor referred toabove represents the flow rate per impeller.

The impeller 28 comprises a plurality of radially-extending blades 32.The number of such blades may vary and generally at least four bladesare employed, with the individual blades being equi-angularly spacedapart. The impeller is illustrated with the blades 32 extendingvertically. However, other orientation of the blades 32 are possible.

Generally, the standpipe 14 has a diameter dimension related to that ofthe impeller 28 and the ratio of the diameter of the standpipe 14 tothat of the impeller 28 generally may vary from about 1:1 to about 2:1.However, the ratio may be lower, if the impeller is mounted below thestandpipe. The impeller 28 generally has a height which corresponds toan approximately 1:1 ratio with its diameter, but the ratio generallymay vary from about 0.3:1 to about 3:1. As the gas is drawn down throughthe standpipe 14 by the action of the rotary impeller 28 and the liquidphase is drawn into the impeller, the action of gas and liquid flows androtary motion produce a vortex of liquid phase in the standpipe and theregion of the impeller 28. Alternatively, the gas may be introducedbelow the impeller and drawn into the interior of the shroud by theaction of the impeller.

The ratio of the projected cross-sectional area of the shrouded impeller28 to the cross-sectional area of the cell may vary widely, and often isless but may be more than in a conventional agitated flotation cell,since the reaction is confined to a small volume of the reaction mediumand will be determined by the ultimate use to which the apparatus 10 isput. The ratio may be as little as about 1:2. However, where additionalprocessing of product is required to be effected efficiently, such asflotation of sulfur, the ratio generally will be higher.

Another function of the impeller 28 is to distribute the induced gasesas bubbles within the liquid medium in the interior of the shroud. Thisresult is achieved by rotation of the impeller 28, resulting in shear ofliquid and gases to form bubbles of relatively wide size distributiondimensioned so that the largest are no more than about 1/2 inch (5 mm)in diameter.

A critical parameter in determining an adequate shearing to form the gasbubbles is the velocity of the outer tip of the blades 32. A blade tipvelocity of at least about 350 in/sec is required to achieve efficient(i.e., 99.99%+) removal of hydrogen sulfide, preferably at least about500, and up to and greater than 700 in/sec This blade tip velocity ismuch higher than typically used in a conventional agitated flotationcell, wherein the maximum velocity is about 275 in/sec.

The impeller 28 is surrounded by a cylindrical stationary shroud 34having a uniform array of circular openings 36 through the wall thereof.The shroud 34 generally has a diameter slightly greater than thestandpipe 14. Although, in the illustrated embodiment, the shroud 34 isright cylindrical and stationary, it is possible for the shroud 34 topossess other shapes. For example, the shroud 34 may be tapered, withthe impeller 28 optionally also being tapered. In addition, the shroud34 may be rotated, if desired, usually in the opposite direction to theimpeller 28. Further, the shroud 34 is shown as a separate element fromthe standpipe 14. However, the shroud 34 may be provided as an extensionof the standpipe, if desired.

Further, the openings 36 in the shroud are illustrated as beingcircular, since this structure is convenient. However, it is possiblefor the openings to have different geometrical shapes, such as square,rectangular or hexagonal. Further, all the openings 36 need not be ofthe same shape or size.

The shroud 34 serves a multiple function in the device. Thus, the shroud34 prevents gases from by-passing the impeller 28, assists in theformation of the vortex in the liquid necessary for gas induction,assists in achieving shearing as well as providing additional shearingand confines the gas-liquid mixture and hence maintains the turbulenceproduced by the impeller 28. The effect of the impeller-shroudcombination may be enhanced by the employment of a series of elongatebaffles, provided on the internal wall of the shroud 34, preferablyvertically extending from the lower end to the upper end of the openingsin the shroud. The gas-liquid mixture flows through and in contact withthe openings36 in the shroud which results in further shearing of thegas bubbles and further intimate contact of the gaseous and liquidphases.

The shroud 34 is spaced only a short distance from the extremity of theimpeller blades 30, in order to provide and promote the above-notedfunctions. Generally, the ratio oft he diameter of the shroud 34 to thatof the impeller 28 generally is about 3:1 to about 1.1:1, preferablyapproximately 1.5:1. The relationship of the shroud and impellerdiameters may be further particularized by the ESI and SEI indicesdiscussed above.

In contrast to the shroud in a conventional agitated flotation cell, theopenings 36 generally are larger in number and smaller in diameter, inorder to provide an increased area for shearing, although an equivalenteffect can be achieved using openings of large aspect ratio, such asslits. When such circular openings are employed, the openings 36generally are uniformly distributed over the wall of the shroud 34 andusually are of equal size. The equivalent diameter of the openings 36often is less than about one inch (25 mm) and generally should be assmall as possible without plugging, preferably about 3/8 to about 5/8inch (about 10 to about 15 mm) in diameter, in order to provide for therequired gas flow therethrough. When the openings 36 are of non-circulargeometrical shape and of aspect ratio which is approximately unity, thenthe area of each such opening 36 generally is, less than the area of acircular opening having an equivalent diameter of about one inch (25mm), preferably about 3/8 to about 5/8 inch (about 10 to about 15 mm).The openings have sharp corners to promote shearing of the gas bubblespassing through the openings and contacting the edges.

The openings 36 are dimensioned to permit a gas flow rate therethroughmeasured at atmospheric pressure corresponding to less than about 0.02lb/min/shroud opening, generally down to about 0.004 lb/min/shroudopening. As noted earlier, the gas flow rate may be higher at or nearthe upper limit of capacity of the unit. Preferably, the gas flow ratethrough the shroud openings is about 0,005 to about 0,007 lb/min/openingin the shroud. As noted above, in general, the gas velocity index is atleast about 18 per second per opening in the shroud, preferably at leastabout 24 per second per opening, and more preferably at least about 30per second per opening.

The shroud 34 is illustrated as extending downwardly for the height ofthe impeller 28. It is possible for the shroud 34 to extend below theheight of the impeller 28 or for less than its full height, if desired.

In addition, in the illustrated embodiment, the impeller 28 is located adistance corresponding to approximately half the diameter of theimpeller 28 from the bottom wall of the reactor 10. It is possible forthis dimension to vary from less than about 0.25:1 to about 1:1 orgreater of the proportion of the diameter dimension of the impeller.This spacing of the impeller 28 from the lower wall allows liquid phaseto be drawn into the area between the impeller 28 and the shroud 34 fromthe mass in the reactor. If desired, a draft tube may be providedextending into the body of the liquid phase from the lower end of theimpeller, to guide liquid into the region of the impeller.

By distributing the gases in the form of bubbles and effecting shearingof the bubbles in contact with the iron chelate solution within theshroud 34 and during passage through the openings 36 therein, rapid masstransfer occurs and the hydrogen sulfide is rapidly oxidized to sulfur.The reaction occurs largely in the immediate region of the impeller 28and shroud 34 and forms sulfur and hydrogen sulfide-depleted gasbubbles.

The sulfur particles initially remain suspended in the turbulentreaction medium but grow in the body of the reaction medium to a sizewhich enables them to be floated by the hydrogen sulfide-depleted gasbubbles. When the sulfur particles have reached a size in the range ofabout 10 to about 50 microns in diameter, they possess sufficientinertia to penetrate the boundary layer of the gas bubbles to therebyenable them to be floated by the upwardly flowing hydrogensulfide-depleted gas bubbles.

Other odiferous components of the hydrogen sulfide-containing gasstream, such as mercaptans, disulfides and odiferous nitrogenouscompounds, such as putrescene and cadaverene, also may be removed byadsorption on the sulfur particles.

At the surface of the aqueous reaction medium, the floated sulfuraccumulates as a froth 38 and the hydrogen sulfide-depleted gas bubblesenter an atmosphere 40 of such gas above the reaction medium 42. Thepresence of the froth 38 tends to inhibit entrainment of an aerosol ofreaction medium in the atmosphere 40.

A hydrogen sulfide-depleted gas flow outlet 44 is provided in the upperclosure 16 to permit the treated gas stream to pass out of the reactorvessel 12.

An adequate freeboard above the liquid level in the reaction vessel isprovided greater than the thickness of the sulfur-laden froth 38, tofurther inhibit aerosol entrainment.

Paddle wheels 46 are provided adjacent the edges of the vessel 12 inoperative relation with the sulfur-laden froth 38, so as to skim thesulfur-laden froth from the surface of the reaction medium 42 intocollecting launder 48 provided at each side of the vessel 12. Theskimmed sulfur is removed periodically or continuously from the launders48 for further processing.

The sulfur is obtained in the form of froth containing about 5 to about30 wt.% sulfur in reaction medium. Since the sulfur is in the form ofparticles of a relatively narrow particle size, the sulfur is readilyseparated from the entrained reaction medium, which is returned to thereactor 10.

The gas-liquid contact apparatus 10 provides a very compact unit whichrapidly and efficiently removes hydrogen sulfide from gas streamscontaining the same. Such gas streams may have a wide range ofconcentrations of hydrogen sulfide. The compact nature of the unit leadsto considerable economies, both in terms of capital cost and operatingcost, when compared to conventional hydrogen sulfide-removal systems.

There has previously been described in U.S. Pat. No. 3,993,563 a gasingestion and mixing device of the general type described herein. Inthat reference, it is indicated that, for the device described therein,if an increase in the-rotor speed is made in an attempt to obtaingreater gas-liquid mixing action, then it is necessary to employ abaffle in the standpipe in order to obtain satisfactory gas ingestion.As is apparent from the description herein, such a baffle is notrequired in the present invention.

However, with larger size units designed to handle large volumes of gas,it may be desirable to provide a conical perforated hood structure abovethe impeller-shroud combination to quieten the surface of the liquidmedium in the vessel.

EXAMPLE Example 1

A pilot plant apparatus was constructed as schematically shown in FIG. 1and was tested for efficiency of removal of hydrogen sulfide from a gasstream containing the same.

The overall liquid capacity of the tank was 135 L. The standpipe had aninside diameter of 71/2 in. (19 cm), and the impeller consisted of sixblades and had a diameter of 51/2 in. (14 cm) and a height of 61/4 in.(16 cm) and was positioned 21/4 in. (5.7 cm) from the base of the tank.

The pilot plant apparatus, fitted with a standard froth flotationdispenser and impeller combination, was charged with 110 L of an aqueoussolution which contained 0.016 mol/L of ethylenediaminetetraacetic acid,iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. ThepH of the aqueous medium was 8.5. The dispenser consisted of astationary cylinder of outside diameter 12 in. (30 cm), height 51/4 in.(14.6 cm) and thickness 3/4 in. (1.9 cm) in which was formed 48 circularopenings each 1.25 in. (3.8 cm) in diameter, for a total circumferentiallength of 188 inches.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 835 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 733 rpm., corresponding to a blade tip velocity of about 211 in/sec.(5.4 m/s). The gas velocity index through the dispenser openings was11.7 per second per opening in the dispenser. (The gas flow rate was0.05 lb/min/opening.) Over the one and a half hour test period, 99.5% ofthe hydrogen sulfide was removed from the gas stream, leaving a residualamount of H₂ S in the gas stream of 20 ppm. Sulphur was formed andappeared as a froth on the surface of the aqueous solution and wasskimmed from the surface using the paddle wheels. Simultaneous removalof hydrogen sulfide from the gas stream and recovery of the sulfurproduced thereby, therefore, was effected.

During the test period, the pH of the aqueous solution dropped to 8.3but no additional alkali was added during this period. Further, noadditional catalyst was added during the period of the test.

Example 2

The procedure of Example 1 was repeated with an increased impellerrotation rate and higher gas flow rate.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 995 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 1772 rpm corresponding to a blade tip velocity of about 510 in/sec.(13 m/s). The gas velocity index through the dispenser openings was 13.7per second per opening in the dispenser. (The gas flow rate was 0.06lb/min/opening.) Over the two hour test period 99.7% of the hydrogensulfide was removed from the gas stream, leaving a residual amount of H₂S of 11 ppm. Sulfur was formed and appeared as a froth on the surface ofthe aqueous solution and was skimmed from the surface. Simultaneousremoval of hydrogen sulfide from the gas stream and recovery of thesulfur produced thereby, therefore, was effected.

During the test period, the pH of the aqueous solution dropped to 8.3but no additional alkali was added during this period. Further, noadditional catalyst was added during this period of the test.

Example 3

The pilot plant apparatus was modified and fitted with a shroud andimpeller combination as illustrated in FIG. 2, was charged with 110 L ofan aqueous solution which contained 0.016 mol/L ofethylenediaminetetraacetic acid, iron-ammonium complex and 0.05 mol/L ofsodium hydrogen carbonate. The pH of the aqueous solution was 8.5. Theshroud consisted of a stationary cylinder of outside diameter 123/4 in.(32.4 cm), height 81/2 in. (21.6 cm), and thickness 1/2 in. (1.3 cm) inwhich was formed 670 openings each of 3/4 in. (1 cm) diameter for atotal circumferential length of 789 inches (20 m). Vertical bafflesextending vertically from top to bottom of the shroud were provided onthe internal wall equally arcuately spaced, ten in number with a 1/4-inch×1/4 -inch (0.6×0.6 cm) space cross section. The impeller wasreplaced by one having a diameter of 61/2 in. (16.5 cm), an increase ofone inch. The other dimensions remained the same. As a result of thesechanges, the self induction volumetric flow rate increased from 14 cfm(396 L/rain) in the unmodified unit to about 30 cfm (850 L/rain).

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 995 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 1754 rpm., corresponding to a blade tip velocity of about 597 in/sec.(15 m/s). The gas velocity index through the shroud was 36.3 per secondper opening. (The gas flow rate was 0.004 lb/min/opening. ) Over the twohour test period 99.998% of the hydrogen sulfide was removed from thegas stream, leaving a residual amount of H₂ S of less than 0.1 ppm.Sulphur was formed and appeared as a froth on the surface of the aqueoussolution and was skimmed from the surface. Simultaneous removal ofhydrogen sulfide from the gas stream and recovery of the sulfur producedthereby, therefore, was effected.

During the test period, the pH of the aqueous solution remainedrelatively constant at 8.5. No additional alkali or catalyst was addedduring the period of this test.

As may be seen from a comparison of the results presented in Examples 1,2 and 3, it is possible to remove hydrogen sulfide with greater than 99%efficiency using an agitated flotation cell which is provided with aconventional dispenser and impeller construction (Examples 1 and 2), asalready described in Canadian Patent No. 1,212,819. However, byemploying a higher blade tip velocity than in the conventional cell, asin Example 2, a modest increase in efficiency can be achieved.

However, as seen in Example 3, with a shroud modified as describedtherein to provide the critical gas flow rate and using the criticalblade tip velocity, efficiency values over 99.99% can be achievedcomprised with that of 99.5% in the conventional unmodified cell,,leaving virtually no residual hydrogen sulfide in the gas stream.

To summarize, 4000 ppm H₂ S in the gas feed can be reduced to residualvalues of 20, 11 and less than 0.1 ppm by the processes illustrated inExamples 1, 2 and 3 respectively.

Example 4

The pilot plant apparatus of FIG. 1 was tested for efficiency of removalof sulfur dioxide from a gas stream containing the same. The elements ofthe pilot plant apparatus were dimensioned as described in Example 3.

The pilot plant apparatus was charged with 110 L of an aqueous slurrycontaining 13.2 kg of CaO and 3450 g of MgSO₄. 7H₂ O. Air, containingvarying amounts of sulfur dioxide was passed through the apparatus viathe standpipe at varying flow rates at room temperature, while theimpeller in the aqueous slurry rotated at a rate varying from 1760 to1770 rpm, corresponding to a blade tip velocity of 599 to 602 in/sec.(15.2 to 15.3 m/s). The corresponding gas velocity indices through theshroud were from 31.1 to 124.5 per Second per opening. (The gas flowrates were 0,003 to 0.01 lb/min/opening.)

A series of one hour runs was performed and the residual SO₂concentration was measured after 45 minutes. The results obtained areset forth in the following Table I:

                  TABLE I                                                         ______________________________________                                        Gas Flow Rate                                                                              SO.sub.2 Concentration                                           (cfm)       In*.sup.(1) (ppmv)                                                                          Out*.sup.(2)                                                                          rpm                                         ______________________________________                                        30          1000          <0.4    1760                                        30          5000          <0.4    1760                                        30          7000          <0.4    1760                                        30          10000         0.6     1760                                        60          900           <0.4    1770                                        75          1000          <0.4    1760                                        100         1000          0.8     1763                                        120         1000          5.6     1770                                        ______________________________________                                         Notes:                                                                        *.sup.(1) Concentration values vary approximately ±10%.                    *.sup.(2) Concentration values vary approximately ±0.2 ppm by volume. 

As may be seen from this data, highly efficient (>99.99%) removal ofsulfur dioxide from the gas stream was obtained using a lime slurry,even at high sulfur dioxide concentrations and less efficient removalwere observed only at high gas flow rate.

Example 5

The procedure of Example 4 was repeated using 110 L of an aqueous slurryof 13.2 kg of ground calcium carbonate and 3450 g of MgSO₄.7H₂ O. Inthese experiments, the impeller was rotated at a speed of 1770 to 1775rpm, corresponding to a blade tip velocity of 602 to 604 in/sec. (15.3to 15.4 m/s). The corresponding gas velocity index through the shroudwere 31.1 to 103.8 per second per opening. (The gas flow rates were0.003 to 0.01 lb/min/opening).

The results obtained are set forth in the following Table II:

                  TABLE II                                                        ______________________________________                                        Gas Flow Rate                                                                              SO.sub.2 Concentration                                           (cfm)       In.sup.(1) (ppmv)                                                                           Out.sup.(2)                                                                           rpm                                         ______________________________________                                        30          900           <0.4    1770                                        30          2000          <0.4    1770                                        30          3000          <0.4    1770                                        30          5000          <0.4    1770                                        30          9000          <0.4    1770                                        30          10000         <0.4    1770                                        45          1000          <0.4    1773                                        60          1000          <0.4    1775                                        75          1050          <0.4    1775                                        100         1000          5.25    1775                                        ______________________________________                                         Notes:                                                                        .sup.(1) Concentration values vary approximately ±10%.                     .sup.(2) Concentration values vary approximately ±0.2 ppm by volume        except for last run, approximately ±1 ppm by volume.                  

As may be seen from this data, highly efficient (>99.99%) removal wasobtained using a ground limestone slurry, even at high sulfur dioxideconcentrations and less efficient removal were observed only at high gasflow rate.

Example 6

A bench scale reaction was set up corresponding in construction to theapparatus of FIG. 1. 4L of the catalyst solution described in Example 3was charged to the reactor. An off-gas stream from a feather cooker of ameat rendering plant was fed to the reactor along with air and, over thetest period, the pH of the catalyst solution, the impeller speed (rpm),the pressure difference between the reactor standpipe and the atmosphereand the temperature of the off-gas stream were all monitored. Gasanalysis for hydrogen sulfide and methanethiol concentrations wereeffected for reactor feed and exit streams.

The separate runs were effected and the results obtained are summarizedin the following Tables III and IV separately:

                  TABLE III                                                       ______________________________________                                                           ▴P                                                                    T    H.sub.2 S.sub.IN                                                                    H.sub.2 S.sub.OUT                                                                     Q                                Time  pH    rpm    "H.sub.2 O                                                                           °C.                                                                         ppmv  ppmv    L/min                            ______________________________________                                        11:20 8.5   2120   -7.6   33   900   --      26                               12:00 8.9   1810   -6.8   30   150   <0.1    20                               13:00 8.9   2060   -7.6   30   33    --      25                               14:00 8.7   2190   -8.2   37   550   <0.1    28                               15:00 8.8   2020   -9.8   40   85    <0.1    31                               16:00 8.7   2060   -7.6   37   1400  <0.1    25                               17:00 8.5   2370   -8.2   50   700   <0.1    30                               ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                                           ▴P                                                                    T    H.sub.2 S.sub.IN                                                                    H.sub.2 S.sub.OUT                                                                     Q                                Time  pH    rpm    "H.sub.2 O                                                                           °C.                                                                         ppmv  ppmv    L/min                            ______________________________________                                        10:10 9.0   2230   -9.2   36   --    --      32                               11:00 8.8   2200   -8.5   44   1100  <0.1    30                               12:00 8.6   2200   -8.4   45   2000  <0.1    29                               13:00 8.6   2200   -8.7   48   7500  <0.1    30                               14:00 8.7   2209   -8.5   49   250   <0.1    30                               15:00 8.4   2270   -7.4   46   200   <0.1    27                               16:00 8.3   2300   -8.2   48   1400  <0.1    29                               17:00 8.5   2320   -7.2   40   85    <0.1    25                               ______________________________________                                         In these Tables, the following abbreviations are used:                        pH: of the catalyst solution                                                  rpm: of the reactor impeller                                                  ▴P: pressure difference between the reactor standpipe and      the atmosphere ("H.sub.2 O)                                                   T: temperature of the slipstream at the point where it is removed from th     feather cooker offgas (°C.)                                            H.sub.2 S.sub.IN : the hydrogen sulphide concentration in the reactor fee     gas stream (ppmv)                                                             H.sub.2 S.sub.OUT : the hydrogen sulphide concentration in the reactor        exit gas stream (ppmv)                                                        Q: the volumetric flow rate of the reactor feed gas stream (L/min)       

During the run described in Table III, the methanethiol concentration at14:00 in the reactor inlet and outlet gas streams was 8 ppmv and <0.1ppmv respectively. When the reactor was stopped at 17:15, the pH of thecatalyst solution was 8.5 and the pressure inside the off-gas duct was-3.2 "H₂ O.

During the run described in Table IV, the methanethiol concentration at12:00 in the reactor inlet and outlet gas streams was 5 ppmv and <0.1ppmv respectively. When the reactor was stopped at 17:00, the pressureinside the off-gas pipe was -5.4 "H₂ O

After approximately 20 minutes of operation from start-up of the rundescribed in Table III, sulfur particles clouded the catalyst solutionand after about one hour a froth layer of elemental sulfur developed onthe surface of the catalyst solution. During the two runs, no sulfur wasremoved from the reactor during the runs except for sulfur suspended insolution when catalyst samples were withdrawn from the reactor. Thesulfur particles increased in size as the tests proceeded as observed bythe length of time required for the sulfur particles to settle in thecatalyst sample removed from the reactor.

As can be seen from the results set forth in Tables III and IV, avariable feed concentration of hydrogen sulfide was decreased to belowthe limit of detection of the test equipment (0.1 ppmv) and theconcentration of highly odiferous methanethiol was below the level ofdetection of the test equipment (0.1 ppmv), and indeed below the levelof detection by the operator.

The highly odiferous off-gas stream contained a variety of nitrogenousand sulfurous organic compounds, in addition to hydrogen sulfide. Thesecompounds which included the methanethiol, probably were removed fromthe gas stream by adsorption on the sulfur particles and the onlydetectable odour in the exit gas stream from the reactor was that ofammonia. This latter observation was confirmed by GC/MS analysis of theinlet and outlet streams, which showed many compounds besides hydrogensulfide in the gas stream entering the reactor which were not detectedin the exiting gas stream.

Example 7

A pilot scale unit was set up corresponding in construction to thatshown in FIG. 1. There were employed an impeller of 13 inches (33 cm)diameter of 123/4 (32.4 cm) inches height, a shroud of outside diameterof 231/2 inches (60.6 cm) and a-height of 13 inches (33 cm), with theshroud having 1824 1/2 -inch (12 mm) diameter openings, and verticalbaffles having 1/2 -inch×1/2 -inch (12 mm×12 mm) cross section.

The modified apparatus was operated to test the mass transfer of oxygenfrom gas phase to liquid phase by employing a readily oxidizablecomponent dissolved in the liquid phase, namely sodium sulfite. Theapparatus was operated at 940 rpm, providing a blade speed velocity of627 in/sec. (16 m/s) and a GVI value of 28.6, 60.2 and 88.0 per secondper opening for three runs (see Table V).

In this procedure, the apparatus first was operated for 10 minutes inthe absence of sodium sulfite and the power requirement (P), standpipepressure (ΔP_(s)), gas flow rate (Q_(z)) and quiecsent water level(L_(s)) after shutdown (to determine liquid volume V_(L)) were measured.1.5 kg of sodium sulfite was added and the reactor operated until anon-zero dissolved oxygen concentration was detected, signifying thatthe sodium sulfite had been consumed.

From the latter time (t), the K,a, i.e. the mass transfer coefficient,may be determined from the relationship:

ΔC.K_(L) a .V_(L).t.=oxygen consumed in Kg (ΔO₂) where ΔC is determinedfrom the equilibrium dissolved oxygen level after completion of the run.ΔC=C₁ -O

where C_(l) is the equilibrium oxygen concentration in the

liquid phase.

Three runs have been performed, as summarized in the

following Table V:

                                      TABLE V                                     __________________________________________________________________________    Q.sub.g   ▴P.sub.s                                                           P  L.sub.g                                                                             C.sub.1                                                                           t   K.sub.L a                                                                         ▴O.sub.2                    (cfm)     ("H.sub.2 O)                                                                      (HP)                                                                             (mm)  (mg/L)                                                                            (secs)                                                                            (hr.sup.-1)                                                                       (kg)                                       __________________________________________________________________________    Run No. 1                                                                           178 0   13.8                                                                             576   8.6 59.2                                                                              1340                                                                              0.203                                                       (1.07 m.sup.3)                                               Run No. 2                                                                           374 4.5 8.0                                                                              484   8.6 61.2                                                                              1300                                                                              0.159                                                       (0.837 m.sup.3)                                              Run No. 3                                                                           553 4.5 7.5                                                                              463   8.6 53.8                                                                              1480                                                                              0.152                                                       (0.799 m.sup.3)                                              __________________________________________________________________________

As may be seen from these results, high levels of mass transfer wereobserved for different flow rates of air to the reactor.

Example 8

This Example illustrates the removal of hydrogen sulfide from gasstreams using ferric hydroxide catalyst. The procedure of Example 3 wasrepeated using the equipment described in Example 7, except that inplace of the iron chelate component of the solution, there was used anaqueous phase having an iron concentration of 0.465 g/L. 1114 L of waterwere heated to 65° C. and 0.0083 mol/L of ferric chloride and 0.025mol/L of sodium hydroxide were added to the heated solution, along with0.05 mol/L of sodium hydrogen carbonate to form a precipitate of ironhydroxide.

Air containing an average of 134 ppmv of hydrogen sulfide and saturatedwith water vapour was passed through the apparatus for a 10-hour periodvia the standpipe at an average volumetric gas flow rate of 407 cfm intocontact with the aqueous medium which had a temperature of about 65° C.throughout the reaction. The pressure drop (ΔP) observed was an averageof 9.1 in H₂ O with a power consumption of 7.0 hp. The impeller wasrotated at an average speed of 942 rpm, corresponding to a blade tipvelocity of 641 in/sec. (16.3 m/s). The average gas velocity indexthrough the shroud was 65.5 per second per opening.

The results obtained are summarized in the following Table VI:

                  TABLE VI                                                        ______________________________________                                               ω T      Q     ▴P (in                                                                   H.sub.2 S.sub.IN                                                                    H.sub.2 S.sub.OUT                 Time   (rpm)   (°C.)                                                                         (acfm)                                                                              H.sub.2 O)                                                                          pH  (ppmv)                                                                              (ppmv)                            ______________________________________                                        10:30  942     64     422   8.9                                               11:30  943     61     426   8.8   9.7 112   5.0                               12:30  941     64     418   8.7       132   3.0                               13:30  941     65     406   8.9   9.8 134   10.0                              14:30  941     65     411   9.1       140   12.0                              15:30  942     65     406   9.1   9.9 138   13.0                              16:30  942     65     406   9.1       136   13.0                              17:30  941     65     406   9.1   9.8 134   13.0                              18:30  941     65     389   9.2       128   14.0                              19:30  942     65     381   9.4   9.8 142   15.0                              20:30  941     66     407   9.4       142   18.0                              ______________________________________                                        Average                                                                              942     65     407   9.1       134                                     ______________________________________                                    

The removal efficiency obtained was not as high as achieved using thechelated iron, as reported in Example 3. The difference in results maybe attributed to a difference in oxidation potential between ironhydroxide and iron EDTA, resulting in a slower rate of reaction. Afurther possibility is that relatively large iron hydroxide flocs wereformed during the batch precipitation procedure, being less ironavailable to function as a catalyst for the reaction. However, it hasnot been possible previously to use iron hydroxide to achieve anysignificant degree of hydrogen sulfide removal. Sulfur produced by theoxidation process was in fine particulate form and had a reddish-browncolour, suggesting adsorption of iron hydroxide onto the sulfurparticles.

Example 9

The procedure of Example 8 was repeated except that the apparatus ofExample 3 was employed. To 110 L of water were added 0.0090 mol/L offerric chloride, 0.027 mol/L of NaOH and 0.05 mol/L of NaHCO₃, to causeprecipitation of iron hydroxide in the aqueous phase in an amount of 0.5g/L of Fe. Air containing varying qualities of hydrogen sulfide waspassed through the apparatus over a 4-hour period at a volumetric gasflow rate of 50 cfm (1416 L/min). The impeller was rotated at a ratecorresponding to an average impeller tip speed of 599 in/sec. (15.2m/s). The average gas velocity index through the shroud was 51.9 persecond per opening.

The results obtained are summarized in the following Table VII:

                  TABLE VII                                                       ______________________________________                                                                  H.sub.2 S.sub.IN                                                                       H.sub.2 S.sub.OUT                          Time        pH            ppm      ppm                                        ______________________________________                                        1:30 pm     (start, T = 9° C.)                                         2:00        10.00         160      0                                          2:30        9.83          210      2                                          3:00        9.72          180      2                                          3:30        9.65          160      4                                          4:00        9.62          170      4                                          (Increased H.sub.2 S.sub.IN)                                                  4:30(T = 17° C.)                                                                   9.55          450      8                                          5:00        9.52          450      9                                          5:30        9.52          450      10                                         ______________________________________                                    

Example 10

The procedure of Example 9 was repeated, except the iron hydroxide forthe oxidation was prepared from ferric sulfite rather than ferricchloride. In this Example, 0.0045 mol/L of Fe₂ (SO₄)₃, 0.027 mol/L ofNaOH and 0.05 mol/L of NaHCO₃ were added to 110 L of water to form anaqueous medium again containing 0.5 g/L Fe.

Air containing varying quantities of hydrogen sulfide was passed throughthe apparatus over a 5-hour period at a volumetric gas flow rate of 50cfm (1416 L/min). The impeller was rotated at a rate corresponding to anaverage impeller tip speed of 599 in/sec. (15.2 m/s). The gas velocityindex through the shroud was 65.5 per second per opening.

The results obtained are summarized in the following Table VIII:

                  TABLE VIII                                                      ______________________________________                                                                H.sub.2 S.sub.IN                                                                      H.sub.2 S.sub.OUT                             Time          pH        ppmv    ppmv                                          ______________________________________                                        11:30         10.24                                                           12:00         9.74      500     3                                             12:30         9.48      600     5                                             13:00         9.43      600     7                                             13:30         9.36      550     8                                             (Increased H.sub.2 S.sub.IN)                                                  14.00         9.10      1550    25                                            14:30         9.19      1100    16                                            15:00         9.19      1150    9                                             15:30         9.14      1150    19                                            (Decreased H.sub.2 S.sub.IN)                                                  16:00         9.20      550     8                                             16:30         9.13      550     9                                             ______________________________________                                    

Higher removal efficiencies were observed in Examples 9 and 10 than inthe case of Example 8. The sulfur produced at the lower temperatureemployed in Examples 9 and 10 were not coated with iron hydroxide, incontrast to Example 8.

Example 11

An experimental unit was set up to test the feasibility of removingsulfur dioxide and hydrogen sulfide from gas streams by a liquid Clausprocess. An experimental unit as described in the aforementioned U.S.Ser. No. 709,158 using two impeller-shroud combination. Sulfur dioxidewas fed to one of the impeller-shroud combinations while hydrogensulfide was fed to the other, under a variety of operating conditions,summarized in the following Table IX:

                  TABLE IX                                                        ______________________________________                                              Sulphite Average   [PO.sub.4.sup.-3 ]                                                                   Average                                                                              Average                                Run # Source   pH        mol/L  [H.sub.2 S] in                                                                       [SO.sub.2 ] in                         ______________________________________                                        1     sat'd    6.89      0.03    400 ppm                                                                             200 ppm                                      NaHSO.sub.3                                                             2     none     7.48      0.08    400 ppm                                                                             200 ppm                                3     sat'd w/ 7.48      0.3     400 ppm                                                                             200 ppm                                      NaHSO.sub.3                                                             4     sat'd w/ 3.7       1.1    5000 ppm                                                                             1000 ppm                                     NaHSO.sub.3                                                             5     60 ml    2.5       1.1    2500 ppm                                                                             500 ppm                                      H.sub.2 SO.sub.3                                                        6     60 ml    2.6       1.1    2500 ppm                                                                             500 ppm                                      H.sub.2 SO.sub.3                                                        7     60 ml    2.7       1.1    5000 ppm                                                                             1000 ppm                                     H.sub.2 SO.sub.3                                                        8     (sol'n)  2.7       1.1    5000 ppm                                                                             1000 ppm                                     (#7)                                                                    ______________________________________                                    

Removal efficiencies attained are summarized in the following Table X:

                  TABLE X                                                         ______________________________________                                               Average Removal (%)                                                                            Average Removal (%)                                   Run #  of H.sub.2 S     of SO.sub.2                                           ______________________________________                                        1      97.2             99.7                                                  2      82.6             99.8                                                  3      83.6             99.8                                                  4      63.6             93.4                                                  5      65.2             93.2                                                  6      59.8             90.0                                                  7      64.1             92.6                                                  8      60.5             90.2                                                  ______________________________________                                    

These results show the effect of pH on removal efficiency, with H₂ S andSO₂ removals being lower at the lower pH levels. These results furthershow that the highest removal efficiency is achieved in the pH range of6.5 to 7.5. However, the preferred range for sulfur production is pH 3.5to 5.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides novelmethod and apparatus for effecting gas-liquid contact for distributionof a gaseous phase in a liquid phase, particularly for the removal ofcomponents from gas streams, such as by chemical reactions or physicalseparation and, if desired, for separating flotable by-products of suchreactions, or for effecting removal of components from the liquid phase.Modifications are possible within the scope of this invention.

What we claim is:
 1. A method for the processing of a viscous liquidwhich is liquid sulfur, bitumen or asphalt, which comprises:immersing arotary impeller comprising a plurality of blades in a mass of saidviscous liquid surrounded by a shroud through which are formed aplurality of openings, and rotating said impeller about a axis at aspeed sufficient to draw viscous liquid from said mass and to flow saidviscous liquid through the openings in said shroud to cause acirculation of said mass of viscous liquid and simultaneously to effectdesorption of an absorbed component in said viscous liquid and/or toeffect transfer of a gaseous component to said viscous liquid.
 2. Themethod of claim 1 wherein a gas is fed to the interior of the shroud andis sheared by said impeller to form a gas-liquid mixture within theshroud, and flowing said gas-liquid mixture through said openings insaid shroud, so as to distribute said gas in said mass of viscousliquid.
 3. The method of claim 2 wherein said viscous liquid has astrippable phase and said gas is capable of effecting stripping of saidstrippable phase.
 4. The method of claim 1 wherein said viscous liquidis liquid sulfur and said gaseous component comprises hydrogen sulfideand an oxidizing gas for oxidizing said hydrogen sulfide is fed to theinterior of the shroud.
 5. A method for the processing of a viscousliquid which is bitumen having a strippable phase which is a volatileorganic material, which comprises:immersing a rotary impeller comprisinga plurality of blades in a mass of said viscous liquid surrounded by ashroud through which are formed a plurality of openings, rotating saidimpeller about a axis at a speed sufficient to draw the viscous liquidfrom said mass and to flow said viscous liquid through the openings insaid shroud to cause a circulation of said mass of viscous liquid,feeding a gas capable of effecting stripping of said strippable phase tothe interior of the shroud and shearing said gas by said impeller toform a gas-liquid mixture with the shroud, and flowing said gas-liquidmixture through said openings in said shroud so as to distribute saidgas in said mass of viscous liquid and to effect desorption of saidstrippable phase by said gas.
 6. The method of claim 5 wherein said gasis an inert gas.
 7. The method of claim 6 wherein said inert gas iscarbon dioxide, nitrogen or an exhaust gas from a combustion engine. 8.A method for the processing of a viscous liquid which is liquid sulfurhaving a strippable phase which is dissolved hydrogen sulfide and/orhydrogen polysulfides, which comprises:immersing a rotary impellercomprising a plurality of blades in a mass of said viscous liquidsurrounded by a shroud through which are formed a plurality of openings,rotating said impeller about a axis at a speed sufficient to draw theviscous liquid from said mass and to flow said viscous liquid throughthe openings in said shroud to cause a circulation of said mass ofviscous liquid, feeding a gas capable of effecting stripping of saidstrippable phase to the interior of the shroud and shearing said gas bysaid impeller to form a gas-liquid mixture within the shroud, andflowing said gas-liquid mixture through said openings in said shroud todistribute said gas in said mass of viscous liquid bed to effectdesorption of said strippable phase by said gas.
 9. The method of claim8 wherein said gas comprises an oxidizing gas for oxidizing saidhydrogen sulfide and/or hydrogen polysulfide to sulfur.
 10. A method forthe processing of a viscous liquid which is liquid sulfur or asphalthaving a dissolved oxidizable phase, which comprises:immersing a rotaryimpeller comprising a plurality of blades in a mass of said viscousliquid surrounded by a shroud through which are formed a plurality ofopenings, rotating said impeller about a axis at a speed sufficient todraw the viscous liquid from said mass and to flow said viscous liquidthrough the openings in said shroud to cause a circulation of said massof viscous feeding an oxidizing gas to the interior of the shroud andshearing said gas by said impeller to form a gas-liquid mixture with theshroud, and flowing said gas-liquid mixture through said openings insaid shroud so as to effect transfer of oxidant to said viscous liquid.11. The method of claim 10 wherein said viscous liquid is liquid sulfur,said liquid sulfur contains hydrogen sulfide and/or hydrogenpolysulfides, and said hydrogen sulfide and/or polysulfides are oxidizedby said oxidizing gas to sulfur.